When a ship or a barge is in a harbor, it may be anchored in open water, or it may be secured to a pier or other stationary structure. Likewise, a ship at an oil well platform is secured to the platform. If the ship is anchored in open water or secured to the pier or platform, the ship or vessel's own gear is used. When securing to a pier of a platform, for example, the ship's lines are brought out from the ship and secured to various kinds of fittings on the stationary structure. The ship's line or vessel's line may have a loop or large eye, spliced into the end of the line which is brought from the ship and secured to the fitting on the stationary structure. Sometimes, both ends of the mooring line are retained aboard the ship and the bight of the line is brought out from the ship and secured to a fitting on the stationary structure. The fitting on the stationary structure may be a bollard, or it may be a sleeve, or it may be a pair of mooring bits, or the fitting on the stationary structure may be a releasable hook. This invention is directed to a releasable hook for retaining a mooring line. The hook may be opened or closed by hydraulic pressure under remote control. In the event of the failure of the power source to energize the hydraulic pressure controlling the hook, then the hook may be released manually.